The Stratigraphic Scheme of the Albertine Graben-East Africa

Abstract

The Albertine Graben is one the most petroliferous onshore rifts in Africa. It forms the northernmost termination of the western arm of the East African Rift System. The sediments within the graben were first studied by Wayland (1925) and O'Brien (1939) among other earlier researchers. These authors used fossil mammals to describe the exposed succession of the graben to be Plio-Pleistocene. Follow up studies particularly by Pickford et al. (1993) introduced most of the stratigraphic units and presented a detailed review of the molluscan and mammalian biostratigraphy of the succession, using correlations to the better-dated mammalian evolutionary lineages of the Kenyan and Ethiopian rift systems. The basic stratigraphic frameworks established by earlier workers have been further developed by the Ugandan Government geoscientists, and international oil companies operating in the graben. Important to note is that the stratigraphic units introduced by the early workers were not fully and formally described, leading to informal and often confusing use of some of the units. Additionally, the early studies were confined to few basins in the graben. Furthermore, the ages of the sediments and correlation between surface and subsurface has been the subject of debate and disagreement. This study therefore aims to establish a coherent stratigraphic scheme for the entire graben by undertaking an extensive integral study of surface and subsurface data involving a review of previous work, study of field exposures, seismic, biostratigraphy and petrophysical studies. It also involves radiometric dating of tuff beds from field exposures for precise age datings. The work precisely describes the type and reference sections for various formations, both in exposure and in the wells drilled in the Graben. It further provides summarized lithostratigraphic columns of the different basins in the graben. The approach reveals that the Semliki area has the most complete sedimentary succession in the graben, spanning the period from early?-middle Miocene to Recent. Models developed from this study suggest that the platform deposits, which are just a small fraction of the thickness of the basinal succession, represent a highly condensed sequence which only saw deposition at times of Lake Highstand. At other times, the platform was marked by non-deposition and sediment bypass, characteristic ironstones that are common in the succession often marking significant discontinuity surfaces.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015